Plasminogen activators (PAs), which convert
plasminogen into the fibrinolytic
protease plasmin, may initiate the degradation of
amyloid-β (Aβ) to suppress the
amyloid pathogenesis. In that way,
tissue plasminogen activator (tPA)-mediated
plasmin activation could maintain a low level of Aβ deposition to delay the pathogenesis of
Alzheimer's disease (AD). In a previous study, we reported that tPA/
plasmin proteolytic activity is attenuated throughout the brain during aging or with Aβ accumulation but clustered intense around the
amyloid plaques in AD brain. The present study demonstrates that the altered proteolytic activity primarily results from the competition between the expressions of tPA and
plasminogen activator inhibitor-1 (PAI-1) in the brains of Tg2576 Aβ-transgenic mice, as revealed by immunohistochemistry and immunoblot assays. Compared with that in the brains of younger Tg2576 mice, tPA
protein is generally reduced throughout the brain in older Tg2576 mice but elevated near
amyloid plaques. In contrary,
PAI-1 expression increases during aging or Aβ deposition with its clusters surrounding
amyloid plaques. No significant alteration in the expression of
urokinase plasminogen activator (uPA) is detected. These results suggest reciprocal feedback influences between tPA,
PAI-1 and Aβ during aging and
amyloid pathogenesis in AD brain; tPA-mediated
plasmin activity is declined throughout the brain causing Aβ deposition during aging, and the Aβ deposits locally attract the cluster of tPA and/or
PAI-1 around their deposits to competitively determine tPA/
plasmin-mediated Aβ proteolysis.